Differences in photoprotective strategy during winter in Eastern white pine and white spruce

Conifers growing in temperate forests utilize sustained forms of thermal dissipation during winter to protect the photosynthetic apparatus from damage, which can be monitored via pronounced reductions in photochemical efficiency (Fv/Fm) during winter. Eastern white pine (Pinus strobus L.) and white spruce (Picea  glauca (Moench) Voss) are known to recover from winter stress at different rates, with pine recovering more slowly than spruce, suggesting different mechanisms for sustained dissipation in these species. Our objectives were to monitor pine and spruce throughout spring recovery in order to provide insights into key mechanisms for sustained dissipation in both species. We measured chlorophyll fluorescence, pigments, and abundance and phosphorylation status of key photosynthetic proteins. We found that both species rely on two forms of sustained dissipation involving retention of high amounts of antheraxanthin (A) + zeaxanthin (Z), one that is very slowly reversible and temperature independent and one that is more dynamic and occurs only on subzero days. Differences in protein abundance suggest that spruce, but not pine, likely upregulates cyclic or alternative pathways of electron transport involving the cytochrome b6f complex and photosystem I (PSI). Both species show an increased sustained phosphorylation of the D1 protein on subzero days, and spruce additionally shows dramatic increases in the sustained phosphorylation of light-harvesting complex II (LHCII) and other PSII core proteins on subzero days only, suggesting that a mechanism of sustained dissipation that is temperature dependent requires sustained phosphorylation of photosynthetic proteins in spruce, possibly allowing for direct energy transfer from PSII to PSI as a mechanism of photoprotection. The data suggest differences in strategy among conifers in mechanisms of sustained thermal dissipation in response to winter stress. Additionally, the flexible induction of sustained A + Z and phosphorylation of photosynthetic proteins in response to subzero temperatures during spring recovery seem to be important in providing photoprotection during transitional periods with high temperature fluctuation.

Seasonal Differences in Leaf Photoprotective Potential between Adults and Juveniles of Two Mediterranean Perennials with Distinct Growth Forms: A Comparative Field Study

The photosynthetic differences between adult and juvenile Mediterranean plants were previously studied under field conditions, yet the corresponding differentiation of their photoprotective efficiency has not been sufficiently investigated. The present study aims to examine possible differences in the photoprotective potential between adults and juveniles of two native Mediterranean plants with distinct growth forms. Thus, the seasonal variations in individual carotenoids, electron transport rate (ETR), and non-photochemical quenching (NPQ) were monitored in fully exposed mature leaves from adults and juveniles of the winter deciduous tree Cercis siliquastrum L. and the evergreen sclerophyllous shrub Nerium oleander L. All plants were grown under apparently similar field conditions. In both species, juveniles displayed substantially lower ETR and increased NPQ values than adults in spring, with the differences intensifying during summer drought and diminishing in autumn. Concomitantly, juveniles showed significantly higher chlorophyll-based total carotenoids in spring and summer mainly due to the higher investment in xanthophyll cycle components (VAZ), in combination with an increased mid-day de-epoxidation state (DEPS) and partial retention of zeaxanthin in the dark. In N. oleander, although ETR was lower in juveniles during winter, NPQ was extremely low in both ages. In conclusion, juveniles exhibit enhanced photoprotection potential, especially in the summer, due to their reduced photochemical capacity. The photosynthetic superiority of adults during the favorable spring period may be attributed to the needs of the co-existing reproductive effort.

Modulation of xanthophyll cycle impacts biomass productivity in the marine microalga Nannochloropsis

Life on earth depends on photosynthetic primary producers that exploit sunlight to fix CO₂ into biomass. Approximately half of global primary production is associated with microalgae living in aquatic environments. Microalgae also represent a promising source of biomass to complement crop cultivation, and they could contribute to the development of a more sustainable bioeconomy. Photosynthetic organisms evolved multiple mechanisms involved in the regulation of photosynthesis to respond to highly variable environmental conditions. While essential to avoid photodamage, regulation of photosynthesis results in dissipation of absorbed light energy, generating a complex trade-off between protection from stress and light-use efficiency. This work investigates the impact of the xanthophyll cycle, the light-induced reversible conversion of violaxanthin into zeaxanthin, on the protection from excess light and on biomass productivity in the marine microalgae of the genus Nannochloropsis. Zeaxanthin is shown to have an essential role in protection from excess light, contributing to the induction of nonphotochemical quenching and scavenging of reactive oxygen species. On the contrary, the overexpression of zeaxanthin epoxidase enables a faster reconversion of zeaxanthin to violaxanthin that is shown to be advantageous for biomass productivity in dense cultures in photobioreactors. These results demonstrate that zeaxanthin accumulation is critical to respond to strong illumination, but it may lead to unnecessary energy losses in light-limiting conditions and accelerating its reconversion to violaxanthin provides an advantage for biomass productivity in microalgae.

Contrasting photoprotective responses of forest trees revealed using PRI light responses sampled with airborne imaging spectrometry

The Photochemical Reflectance Index (PRI) provides an optical indicator of photosynthetic light-use efficiency, photoprotection, and stress in plants. Although PRI can be applied in remote sensing, its interpretation depends on irradiance, which is hard to obtain from satellite or airborne imagery. To quantify forest photoprotective responses remotely, we developed a framework for modeling and interpreting PRI-light responses of individual trees and species using airborne imaging spectrometry coupled with georeferenced forest inventory data from a temperate broad-leaved forest. We derived an irradiance proxy, used hierarchical modeling to analyze PRI-light responses, and developed a framework of physiological interpretations of model parameters as facultative and constitutive components of photoprotection. Photochemical Reflectance Index declined with illumination, and PRI-light relationships varied with landscape position and among tree crowns and species. More sun-exposed foliage had lower intercepts and slopes of the relationship, indicating greater constitutive, but less facultative, photoprotection. We show that tree photoprotective strategies can be quantified at multiple scales using airborne hyperspectral data in structurally complex forests. Our findings and approach have important implications for the remote sensing of forest stress by offering a new way to assess functional diversity through dynamic differences in photoprotection and photosynthetic downregulation and providing previsual indicators of forest stress.

A wider spectrum of avoidance and tolerance mechanisms explained ozone sensitivity of two white poplar ploidy levels

BACKGROUND AND AIMS

Polyploidization can improve plant mass yield for bioenergy support, yet few studies have investigated ozone (O3) sensitivity linked to internal regulatory mechanisms at different ploidy levels.

METHODS

Diploid and triploid Populus tomentosa plants were exposed to ambient and ambient plus 60 ppb [O3]. We explored their differences in sensitivity (leaf morphological, physiological and biochemical traits, and plant mass) as well as mechanisms of avoidance (stomatal conductance, xanthophyll cycle, thermal dissipation) and tolerance (ROS scavenging system) in response to O3 at two developmental phases.

KEY RESULTS

Triploid plants had the highest plant growth under ambient O3, even under O3 fumigation. However, triploid plants were the most sensitive to O3 and under elevated O3 showed the largest decreases in photosynthetic capacity and performance, as well as increased shoot:root ratio, and the highest lipid peroxidation. Thus, plant mass production could be impacted in triploid plants under long-term O3 contamination. Both diploid and triploid plants reduced stomatal aperture in response to O3, thereby reducing O3 entrance, yet only in diploid plants was reduced stomatal aperture associated with minimal (non-significant) damage to photosynthetic pigments and lower lipid peroxidation.

CONCLUSIONS

Tolerance mechanisms of plants of both ploidy levels mainly focused on the enzymatic reduction of hydrogen peroxide through catalase and peroxidase, yet these homeostatic regulatory mechanisms were higher in diploid plants. Our study recommends triploid white poplar as a bioenergy species only under short-term O3 contamination. Under continuously elevated O3 over the long term, diploid white poplar may perform better.

Improving photosynthetic efficiency by modulating non-photochemical quenching

High light exposure rapidly activates non-photochemical quenching (NPQ), protecting plants from photooxidative damage. Contrarily, its relaxation upon transition to normal light occurs quite slowly, limiting photosynthetic efficiency. De Souza et al. demonstrated that by overexpressing NPQ-related genes, faster NPQ relaxation and enhanced photosynthesis can be achieved under fluctuating light conditions.

Isolation of fucoxanthin chlorophyll protein complexes of the centric diatom Thalassiosira pseudonana associated with the xanthophyll cycle enzyme diadinoxanthin de-epoxidase

In the present study, low concentrations of the very mild detergent n-dodecyl-α-d-maltoside in conjunction with sucrose gradient ultracentrifugation were used to prepare fucoxanthin chlorophyll protein (FCP) complexes of the centric diatom Thalassiosira pseudonana. Two main FCP fractions were observed in the sucrose gradients, one in the upper part and one at high sucrose concentrations in the lower part of the gradient. The first fraction was dominated by the 18 kDa FCP protein band in SDS-gels. Since this fraction also contained other protein bands, it was designated as fraction enriched in FCP-A complex. The second fraction contained mainly the 21 kDa FCP band, which is typical for the FCP-B complex. Determination of the lipid composition showed that both FCP fractions contained monogalactosyl diacylglycerol as the main lipid followed by the second galactolipid of the thylakoid membrane, namely digalactosyl diacylglycerol. The negatively charged lipids sulfoquinovosyl diacylglycerol and phosphatidyl glycerol were also present in both fractions in pronounced concentrations. With respect to the pigment composition, the fraction enriched in FCP-A contained a higher amount of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (Dt), whereas the FCP-B fraction was characterized by a lower ratio of xanthophyll cycle pigments to the light-harvesting pigment fucoxanthin. Protein analysis by mass spectrometry revealed that in both FCP fractions the xanthophyll cycle enzyme diadinoxanthin de-epoxidase (DDE) was present. In addition, the analysis showed an enrichment of DDE in the fraction enriched in FCP-A but only a very low amount of DDE in the FCP-B fraction. In-vitro de-epoxidation assays, employing the isolated FCP complexes, were characterized by an inefficient conversion of DD to Dt. However, in line with the heterogeneous DDE distribution, the fraction enriched in FCP-A showed a more pronounced DD de-epoxidation compared with the FCP-B.

Green diatom mutants reveal an intricate biosynthetic pathway of fucoxanthin

Fucoxanthin is a major light-harvesting pigment in ecologically important algae such as diatoms, haptophytes, and brown algae (Phaeophyceae). Therefore, it is a major driver of global primary productivity. Species of these algal groups are brown colored because the high amounts of fucoxanthin bound to the proteins of their photosynthetic machineries enable efficient absorption of green light. While the structure of these fucoxanthin-chlorophyll proteins has recently been resolved, the biosynthetic pathway of fucoxanthin is still unknown. Here, we identified two enzymes central to this pathway by generating corresponding knockout mutants of the diatom Phaeodactylum tricornutum that are green due to the lack of fucoxanthin. Complementation of the mutants with the native genes or orthologs from haptophytes restored fucoxanthin biosynthesis. We propose a complete biosynthetic path to fucoxanthin in diatoms and haptophytes based on the carotenoid intermediates identified in the mutants and in vitro biochemical assays. It is substantially more complex than anticipated and reveals diadinoxanthin metabolism as the central regulatory hub connecting the photoprotective xanthophyll cycle and the formation of fucoxanthin. Moreover, our data show that the pathway evolved by repeated duplication and neofunctionalization of genes for the xanthophyll cycle enzymes violaxanthin de-epoxidase and zeaxanthin epoxidase. Brown algae lack diadinoxanthin and the genes described here and instead use an alternative pathway predicted to involve fewer enzymes. Our work represents a major step forward in elucidating the biosynthesis of fucoxanthin and understanding the evolution, biogenesis, and regulation of the photosynthetic machinery in algae.

High non-photochemical quenching of VPZ transgenic potato plants limits CO2 assimilation under high light conditions and reduces tuber yield under fluctuating light

Under natural conditions, photosynthesis has to be adjusted to fluctuating light intensities. Leaves exposed to high light dissipate excess light energy in form of heat at photosystem II (PSII) by a process called non-photochemical quenching (NPQ). Upon fast transition from light to shade, plants lose light energy by a relatively slow relaxation from photoprotection. Combined overexpression of violaxanthin de-epoxidase (VDE), PSII subunit S (PsbS) and zeaxanthin epoxidase (ZEP) in tobacco accelerates relaxation from photoprotection, and increases photosynthetic productivity. In Arabidopsis, expression of the same three genes (VPZ) resulted in a more rapid photoprotection but growth of the transgenic plants was impaired. Here we report on VPZ expressing potato plants grown under various light regimes. Similar to tobacco and Arabidopsis, induction and relaxation of NPQ was accelerated under all growth conditions tested, but did not cause an overall increased photosynthetic rate or growth of transgenic plants. Tuber yield of VPZ expressing plants was unaltered as compared to control plants under constant light conditions and even decreased under fluctuating light conditions. Under control conditions, levels of the phytohormone abscisic acid (ABA) were found to be elevated, indicating an increased violaxanthin availability in VPZ plants. However, the increased basal ABA levels did not improve drought tolerance of VPZ transgenic potato plants under greenhouse conditions. The failure to benefit from improved photoprotection is most likely caused by a reduced radiation use efficiency under high light conditions resulting from a too strong NPQ induction. Mitigating this negative effect in the future might help to improve photosynthetic performance in VPZ expressing potato plants.

Non-Photochemical Quenching under Drought and Fluctuating Light

Plants grow in a variable environment in regard to soil water and light driving photochemical reactions. Light energy exceeding plant capability to use it for photochemical reactions must be dissipated by processes of non-photochemical quenching (NPQ). The aim of the study was to evaluate the impact of various components of NPQ on the response of Arabidopsis thaliana to fluctuating light and water availability. A laboratory experiment with Arabidopsis thaliana wild type (WT) and mutants npq1 and npq4 grown under optimum or reduced water availability was conducted. Dark-adapted plants were illuminated with fluctuating light (FL) of two intensities (55 and 530 μmol m⁻² s⁻¹) with each of the phases lasting for 20 s. The impact of water availability on the role of zeaxanthin and PsbS protein in NPQ induced at FL was analysed. The water deficit affected the dynamics of NPQ induced by FL. The lack of zeaxanthin or PsbS reduced plant capability to cope with FL. The synergy of both of these components was enhanced in regard to the amplitude of NPQ in the drought conditions. PsbS was shown as a component of primary importance in suiting plant response to FL under optimum and reduced water availability.

The Loroxanthin Cycle: A New Type of Xanthophyll Cycle in Green Algae (Chlorophyta)

Xanthophyll cycles (XC) have proven to be major contributors to photoacclimation for many organisms. This work describes a light-driven XC operating in the chlorophyte Chlamydomonas reinhardtii and involving the xanthophylls Lutein (L) and Loroxanthin (Lo). Pigments were quantified during a switch from high to low light (LL) and at different time points from cells grown in Day/Night cycle. Trimeric LHCII was purified from cells acclimated to high or LL and their pigment content and spectroscopic properties were characterized. The Lo/(L + Lo) ratio in the cells varies by a factor of 10 between cells grown in low or high light (HL) leading to a change in the Lo/(L + Lo) ratio in trimeric LHCII from .5 in low light to .07 in HL. Trimeric LhcbMs binding Loroxanthin have 5 ± 1% higher excitation energy (EE) transfer (EET) from carotenoid to Chlorophyll as well as higher thermo- and photostability than trimeric LhcbMs that only bind Lutein. The Loroxanthin cycle operates on long time scales (hours to days) and likely evolved as a shade adaptation. It has many similarities with the Lutein-epoxide - Lutein cycle (LLx) of plants.

Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum

Photosynthetic organisms in nature often experience light fluctuations. While low light conditions limit the energy uptake by algae, light absorption exceeding the maximal rate of photosynthesis may go along with enhanced formation of potentially toxic reactive oxygen species. To preempt high light-induced photodamage, photosynthetic organisms evolved numerous photoprotective mechanisms. Among these, energy-dependent fluorescence quenching (qE) provides a rapid mechanism to dissipate thermally the excessively absorbed energy. Diatoms thrive in all aquatic environments and thus belong to the most important primary producers on earth. qE in diatoms is provided by a concerted action of Lhcx proteins and the xanthophyll cycle pigment diatoxanthin. While the exact Lhcx activation mechanism of diatom qE is unknown, two lumen-exposed acidic amino acids within Lhcx proteins were proposed to function as regulatory switches upon light-induced lumenal acidification. By introducing a modified Lhcx1 lacking these amino acids into a Phaeodactylum tricornutum Lhcx1-null qE knockout line, we demonstrate that qE is unaffected by these two amino acids. Based on sequence comparisons with Lhcx4, being incapable of providing qE, we perform domain swap experiments of Lhcx4 with Lhcx1 and identify two peptide motifs involved in conferring qE. Within one of these motifs, we identify a tryptophan residue with a major influence on qE establishment. This tryptophan residue is located in close proximity to the diadinoxanthin/diatoxanthin-binding site based on the recently revealed diatom Lhc crystal structure. Our findings provide a structural explanation for the intimate link of Lhcx and diatoxanthin in providing qE in diatoms.

ROS-Scavengers, Osmoprotectants and Violaxanthin De-Epoxidation in Salt-Stressed Arabidopsis thaliana with Different Tocopherol Composition

To determine the role of α- and γ-tocopherol (TC), this study compared the response to salt stress (200 mM NaCl) in wild type (WT) Arabidopsis thaliana (L.) Heynh. And its two mutants: (1) totally TC-deficient vte1; (2) vte4 accumulating γ-TC instead of α-TC; and (3) tmt transgenic line overaccumulating α-TC. Raman spectra revealed that salt-exposed α-TC accumulating plants were more flexible in regulating chlorophyll, carotenoid and polysaccharide levels than TC deficient mutants, while the plants overaccumulating γ-TC had the lowest levels of these biocompounds. Tocopherol composition and NaCl concentration affected xanthophyll cycle by changing the rate of violaxanthin de-epoxidation and zeaxanthin formation. NaCl treated plants with altered TC composition accumulated less oligosaccharides than WT plants. α-TC deficient plants increased their oligosaccharide levels and reduced maltose amount, while excessive accumulation of α-TC corresponded with enhanced amounts of maltose. Salt-stressed TC-deficient mutants and tmt transgenic line exhibited greater proline levels than WT plants, lower chlorogenic acid levels, and lower activity of catalase and peroxidases. α-TC accumulating plants produced more methylated proline- and glycine- betaines, and showed greater activity of superoxide dismutase than γ-TC deficient plants. Under salt stress, α-TC demonstrated a stronger regulatory effect on carbon- and nitrogen-related metabolites reorganization and modulation of antioxidant patterns than γ-TC. This suggested different links of α- and γ-TCs with various metabolic pathways via various functions and metabolic loops.

Mechanisms shaping the synergism of zeaxanthin and PsbS in photoprotective energy dissipation in the photosynthetic apparatus of plants

Safe operation of photosynthesis is vital to plants and is ensured by the activity of processes protecting chloroplasts against photo-damage. The harmless dissipation of excess excitation energy is considered to be the primary photoprotective mechanism and is most effective in the combined presence of PsbS protein and zeaxanthin, a xanthophyll accumulated in strong light as a result of the xanthophyll cycle. Here we address the problem of specific molecular mechanisms underlying the synergistic effect of zeaxanthin and PsbS. The experiments were conducted with Arabidopsis thaliana, using wild-type plants, mutants lacking PsbS (npq4), and mutants affected in the xanthophyll cycle (npq1), with the application of molecular spectroscopy and imaging techniques. The results lead to the conclusion that PsbS interferes with the formation of densely packed aggregates of thylakoid membrane proteins, thus allowing easy exchange and incorporation of xanthophyll cycle pigments into such structures. It was found that xanthophylls trapped within supramolecular structures, most likely in the interfacial protein region, determine their photophysical properties. The structures formed in the presence of violaxanthin are characterized by minimized dissipation of excitation energy. In contrast, the structures formed in the presence of zeaxanthin show enhanced excitation quenching, thus protecting the system against photo-damage.

Features of the Duckweed Lemna That Support Rapid Growth under Extremes of Light Intensity

This study addresses the unique functional features of duckweed via comparison of Lemna gibba grown under controlled conditions of 50 versus 1000 µmol photons m-2 s-1 and of a L. minor population in a local pond with a nearby population of the biennial weed Malva neglecta. Principal component analysis of foliar pigment composition revealed that Malva was similar to fast-growing annuals, while Lemna was similar to slow-growing evergreens. Overall, Lemna exhibited traits reminiscent of those of its close relatives in the family Araceae, with a remarkable ability to acclimate to both deep shade and full sunlight. Specific features contributing to duckweed's shade tolerance included a foliar pigment composition indicative of large peripheral light-harvesting complexes. Conversely, features contributing to duckweed's tolerance of high light included the ability to convert a large fraction of the xanthophyll cycle pool to zeaxanthin and dissipate a large fraction of absorbed light non-photochemically. Overall, duckweed exhibited a combination of traits of fast-growing annuals and slow-growing evergreens with foliar pigment features that represented an exaggerated version of that of terrestrial perennials combined with an unusually high growth rate. Duckweed's ability to thrive under a wide range of light intensities can support success in a dynamic light environment with periodic cycles of rapid expansion.

Photosynthesis and photoprotection characteristics related to ROS production in three Chattonella (Raphidophyceae) species

The toxic potential of Chattonella is associated with a high production of reactive oxygen species (ROS). Chattonella species can tolerate high irradiance levels but seems not to be efficient in the induction of nonphotochemical chl a fluorescence (NPQ) under light stress conditions. Therefore, we postulated that high ROS production of this microalgal group is related to the lack of effective photoprotection mechanisms. We compared the NPQ induction, xanthophyll cycle interconversion (XC), and the production of the ion superoxide (O₂^- ) in Chattonella marina var. antiqua, Chattonella sp., and C. marina acclimated to 43 (LL) and 300 µmol photons · m²  · s^-1 (HL). We also evaluated the photosynthetic characteristics of the three strains. Photosynthesis saturated at relative high irradiances (above 500 µmol photons · m²  · s^-1 ) in LL and HL Chattonella strains. For the first time, we documented the conversion of diadinoxanthin into diatoxanthin in microalgae that have violaxanthin as the major XC carotenoid. The slow NPQ induction indicated that qE (fast component of NPQ) was not present, and this process was related to the interconversion of XC pigments. However, the quenching efficiency (QE) of deepoxidated xanthophylls was low in the three Chattonella strains. The strain with the lowest QE produced the highest amount of a O₂^- . Therefore, ROS production in Chattonella seems to be related to a low expression of XC-related thermal PSII dissipation.

The desert green algae Chlorella ohadii thrives at excessively high light intensities by exceptionally enhancing the mechanisms that protect photosynthesis from photoinhibition

Although light is the driving force of photosynthesis, excessive light can be harmful. One of the main processes that limits photosynthesis is photoinhibition, the process of light-induced photodamage. When the absorbed light exceeds the amount that is dissipated by photosynthetic electron flow and other processes, damaging radicals are formed that mostly inactivate photosystem II (PSII). Damaged PSII must be replaced by a newly repaired complex in order to preserve full photosynthetic activity. Chlorella ohadii is a green microalga, isolated from biological desert soil crusts, that thrives under extreme high light and is highly resistant to photoinhibition. Therefore, C. ohadii is an ideal model for studying the molecular mechanisms underlying protection against photoinhibition. Comparison of the thylakoids of C. ohadii cells that were grown under low light versus extreme high light intensities found that the alga employs all three known photoinhibition protection mechanisms: (i) massive reduction of the PSII antenna size; (ii) accumulation of protective carotenoids; and (iii) very rapid repair of photodamaged reaction center proteins. This work elucidated the molecular mechanisms of photoinhibition resistance in one of the most light-tolerant photosynthetic organisms, and shows how photoinhibition protection mechanisms evolved to marginal conditions, enabling photosynthesis-dependent life in severe habitats.

Shedding light on the dark side of xanthophyll cycles

Xanthophyll cycles are broadly important in photoprotection, and the reversible de-epoxidation of xanthophylls typically occurs in excess light conditions. However, as presented in this review, compiling evidence in a wide range of photosynthetic eukaryotes shows that xanthophyll de-epoxidation also occurs under diverse abiotic stress conditions in darkness. Light-driven photochemistry usually leads to the pH changes that activate de-epoxidases (e.g. violaxanthin de-epoxidase), but in darkness alternative electron transport pathways and luminal domains enriched in monogalactosyl diacyl glycerol (which enhance de-epoxidase activity) likely enable de-epoxidation. Another 'dark side' to sustaining xanthophyll de-epoxidation is inactivation and/or degradation of epoxidases (e.g. zeaxanthin epoxidase). There are obvious benefits of such activity regarding stress tolerance, and indeed this phenomenon has only been reported in stressful conditions. However, more research is required to unravel the mechanisms and understand the physiological roles of dark-induced formation of zeaxanthin. Notably, the de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin in darkness is still a frequently ignored process, perhaps because it questions a previous paradigm. With that in mind, this review seeks to shed some light on the dark side of xanthophyll de-epoxidation, and point out areas for future work.

Leaf photosynthetic and anatomical insights into mechanisms of acclimation in rice in response to long-term fluctuating light

Long-term fluctuating light (FL) conditions are very common in natural environments. The physiological and biochemical mechanisms for acclimation to FL differ between species. However, most of the current conclusions regarding acclimation to FL were made based on studies in algae or Arabidopsis thaliana. It is still unclear how rice (Oryza sativa L.) integrate multiple physiological changes to acclimate to long-term FL. In this study, we found that rice growth was repressed under long-term FL. By systematically measuring phenotypes and physiological parameters, we revealed that: (a) under short-term FL, photosystem I (PSI) was inhibited, while after 1-7 days of long-term FL, both PSI and PSII were inhibited. Higher acceptor-side limitation in electron transport and higher overall nonphotochemical quenching (NPQ) explained the lower efficiencies of PSI and PSII, respectively. (b) An increase in pH differences across the thylakoid membrane and a decrease in thylakoid proton conductivity revealed a reduction of ATP synthase activity. (c) Using electron microscopy, we showed a decrease in membrane stacking and stomatal opening after 7 days of FL treatment. Taken together, our results show that electron flow, ATP synthase activity and NPQ regulation are the major processes determining the growth performance of rice under long-term FL conditions.

Zeaxanthin, a Molecule for Photoprotection in Many Different Environments

Conversion of sunlight into photochemistry depends on photoprotective processes that allow safe use of sunlight over a broad range of environmental conditions. This review focuses on the ubiquity of photoprotection associated with a group of interconvertible leaf carotenoids, the xanthophyll cycle. We survey the striking plasticity of this process observed in nature with respect to (1) xanthophyll cycle pool size, (2) degree and speed of interconversion of its components, and (3) flexibility in the association between xanthophyll cycle conversion state and photoprotective dissipation of excess excitation energy. It is concluded that the components of this system can be independently tuned with a high degree of flexibility to produce a fit for different environments with various combinations of light, temperature, and other factors. In addition, the role of genetic variation is apparent from variation in the response of different species growing side-by-side in the same environment. These findings illustrate how field studies can generate insight into the adjustable levers that allow xanthophyll cycle-associated photoprotection to support plant photosynthetic productivity and survival in environments with unique combinations of environmental factors.

Evolutionary divergence of photoprotection in the green algal lineage: a plant-like violaxanthin de-epoxidase enzyme activates the xanthophyll cycle in the green alga Chlorella vulgaris modulating photoprotection

The xanthophyll cycle is the metabolic process by which the carotenoid violaxanthin is de-epoxidated to zeaxanthin, a xanthophyll with a crucial photoprotective role in higher plants and mosses. The role of zeaxanthin is still unclear in green algae, and a peculiar violaxanthin de-epoxidating enzyme was found in the model organism Chlamydomonas reinhardtii. Here, we investigated the molecular details and functions of the xanthophyll cycle in the case of Chlorella vulgaris, one of the green algae most considered for industrial cultivation, where resistance to high light stress is a prerequisite for sustainable biomass production. Identification of the violaxanthin de-epoxidase enzyme in C. vulgaris was performed by genome mining and in vitro analysis of the catalytic activity of the gene product identified. The photoprotective role of zeaxanthin was then investigated in vivo and in isolated pigment-binding complexes. The results obtained demonstrate the functioning, even though with a different pH sensitivity, of a plant-like violaxanthin de-epoxidase enzyme in C. vulgaris. Differently from C. reinhardtii, zeaxanthin accumulation in C. vulgaris was found to be crucial for photoprotective quenching of excitation energy harvested by both photosystem I and II. These findings demonstrate an evolutionary divergence of photoprotective mechanisms among Chlorophyta.

Zeaxanthin and Lutein: Photoprotectors, Anti-Inflammatories, and Brain Food

This review compares and contrasts the role of carotenoids across the taxa of life—with a focus on the xanthophyll zeaxanthin (and its structural isomer lutein) in plants and humans. Xanthophylls’ multiple protective roles are summarized, with attention to the similarities and differences in the roles of zeaxanthin and lutein in plants versus animals, as well as the role of meso-zeaxanthin in humans. Detail is provided on the unique control of zeaxanthin function in photosynthesis, that results in its limited availability in leafy vegetables and the human diet. The question of an optimal dietary antioxidant supply is evaluated in the context of the dual roles of both oxidants and antioxidants, in all vital functions of living organisms, and the profound impact of individual and environmental context.

Lipid Dependence of Xanthophyll Cycling in Higher Plants and Algae

The xanthophyll cycles of higher plants and algae represent an important photoprotection mechanism. Two main xanthophyll cycles are known, the violaxanthin cycle of higher plants, green and brown algae and the diadinoxanthin cycle of Bacillariophyceae, Xanthophyceae, Haptophyceae, and Dinophyceae. The forward reaction of the xanthophyll cycles consists of the enzymatic de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin or diadinoxanthin to diatoxanthin during periods of high light illumination. It is catalyzed by the enzymes violaxanthin or diadinoxanthin de-epoxidase. During low light or darkness the back reaction of the cycle, which is catalyzed by the enzymes zeaxanthin or diatoxanthin epoxidase, restores the epoxidized xanthophylls by a re-introduction of the epoxy groups. The de-epoxidation reaction takes place in the lipid phase of the thylakoid membrane and thus, depends on the nature, three dimensional structure and function of the thylakoid lipids. As the xanthophyll cycle pigments are usually associated with the photosynthetic light-harvesting proteins, structural re-arrangements of the proteins and changes in the protein-lipid interactions play an additional role for the operation of the xanthophyll cycles. In the present review we give an introduction to the lipid and fatty acid composition of thylakoid membranes of higher plants and algae. We introduce the readers to the reaction sequences, enzymes and function of the different xanthophyll cycles. The main focus of the review lies on the lipid dependence of xanthophyll cycling. We summarize the current knowledge about the role of lipids in the solubilization of xanthophyll cycle pigments. We address the importance of the three-dimensional lipid structures for the enzymatic xanthophyll conversion, with a special focus on non-bilayer lipid phases which are formed by the main thylakoid membrane lipid monogalactosyldiacylglycerol. We additionally describe how lipids and light-harvesting complexes interact in the thylakoid membrane and how these interactions can affect the structure of the thylakoids. In a dedicated chapter we offer a short overview of current membrane models, including the concept of membrane domains. We then use these concepts to present a model of the operative xanthophyll cycle as a transient thylakoid membrane domain which is formed during high light illumination of plants or algal cells.

Plant Origin, but Not Phylogeny, Drive Species Ecophysiological Response to Projected Climate

Knowledge of the relationship between environmental conditions and species traits is an important prerequisite for understanding determinants of community composition and predicting species response to novel climatic conditions. Despite increasing number of studies on this topic, our knowledge on importance of genetic differentiation, plasticity and their interactions along larger sets of species is still limited especially for traits related to plant ecophysiology. We studied variation in traits related to growth, leaf chemistry, contents of photosynthetic pigments and activity of antioxidative enzymes, stomata morphology and photosynthetic activity across eight Impatiens species growing along altitudinal gradients in Himalayas cultivated in three different temperature regimes and explored effects of among species phylogenetic relationships on the results. Original and target climatic conditions determine trait values in our system. The traits are either highly plastic (e.g., APX, CAT, plant size, neoxanthin, β-carotene, chlorophyll a/b, DEPSC) or are highly differentiated among populations (stomata density, lutein production). Many traits show strong among population differentiation in degree of plasticity and direction in response to environmental changes. Most traits indicate that the species will profit from the expected warming. This suggests that different processes determine the values of the different traits and separating the importance of genetic differentiation and plasticity is crucial for our ability to predict species response to future climate changes. The results also indicate that evolution of the traits is not phylogenetically constrained but including phylogenetic information into the analysis may improve our understanding of the trait-environment relationships as was apparent from the analysis of SLA.

Spatial and Temporal Variability of Plant Leaf Responses Cascade after PSII Inhibition: Raman, Chlorophyll Fluorescence and Infrared Thermal Imaging

The use of photosystem II (PSII) inhibitors allows simulating cascade of defense and damage responses, including the oxidative stress. In our study, PSII inhibiting herbicide metribuzin was applied to the leaf of the model plant species Chenopodium album. The temporally and spatially resolved cascade of defense responses was studied noninvasively at the leaf level by combining three imaging approaches: Raman spectroscopy as a principal method, corroborated by chlorophyll a fluorescence (ChlF) and infrared thermal imaging. ChlF imaging show time-dependent transport in acropetal direction through veins and increase of area affected by metribuzin and demonstrated the ability to distinguish between fast processes at the level of electron transport (1 - Vj) from slow processes at the level of non-photochemical energy dissipation (NPQ) or maximum efficiency of PSII photochemistry (Fv/Fm). The high-resolution resonance Raman images show zones of local increase of carotenoid signal 72 h after the herbicide application, surrounding the damaged tissue, which points to the activation of defense mechanisms. The shift in the carotenoid band indicates structural changes in carotenoids. Finally, the increase of leaf temperature in the region surrounding the spot of herbicide application and expanding in the direction to the leaf tip proves the metribuzin effect on slow stomata closure.

Linking Leaf Water Potential, Photosynthesis and Chlorophyll Loss With Mechanisms of Photo- and Antioxidant Protection in Juvenile Olive Trees Subjected to Severe Drought

The identification of drought-tolerant olive tree genotypes has become an urgent requirement to develop sustainable agriculture in dry lands. However, physiological markers linking drought tolerance with mechanistic effects operating at the cellular level are still lacking, in particular under severe stress, despite the urgent need to develop these tools in the current frame of global change. In this context, 1-year-old olive plants growing in the greenhouse and with a high intra-specific variability (using various genotypes obtained either from cuttings or seeds) were evaluated for drought tolerance under severe stress. Growth, plant water status, net photosynthesis rates, chlorophyll contents and the extent of photo- and antioxidant defenses (including the de-epoxidation state of the xanthophyll cycle, and the contents of carotenoids and vitamin E) were evaluated under well-watered conditions and severe stress (by withholding water for 60 days). Plants were able to continue photosynthesizing under severe stress, even at very low leaf water potential of -4 to -6 MPa. This ability was achieved, at least in part, by the activation of photo- and antioxidant mechanisms, including not only increased xanthophyll cycle de-epoxidation, but also enhanced α-tocopherol contents. "Zarrazi" (obtained from seeds) and "Chemlali" (obtained from cuttings) showed better performance under severe water stress compared to the other genotypes, which was associated to their ability to trigger a higher antioxidant protection. It is concluded that (i) drought tolerance among the various genotypes tested is associated with antioxidant protection in olive trees, (ii) the extent of xanthophyll cycle de-epoxidation is strongly inversely related to photosynthetic rates, and (iii) vitamin E accumulation is sharply induced upon severe chlorophyll degradation.

The zeaxanthin epoxidase is degraded along with the D1 protein during photoinhibition of photosystem II

The xanthophyll zeaxanthin is synthesized in chloroplasts upon high light exposure of plants and serves central photoprotective functions. The reconversion of zeaxanthin to violaxanthin is catalyzed by the zeaxanthin epoxidase (ZEP). ZEP shows highest activity after short and moderate high light periods, but becomes gradually down-regulated in response to increasing high light stress along with down-regulation of photosystem II (PSII) activity. ZEP activity and ZEP protein levels were studied in response to high light stress in four plant species: Arabidopsis thaliana, Pisum sativum, Nicotiana benthamiana and Spinacia oleracea. In all species, ZEP protein was degraded during photoinhibition of PSII in parallel with the D1 protein of PSII. In the presence of streptomycin, an inhibitor of chloroplast protein synthesis, photoinhibition of PSII and ZEP activity as well as degradation of D1 and ZEP protein was strongly increased, indicating a close correlation of ZEP regulation with PSII photoinhibition and repair. The concomitant high light-induced inactivation/degradation of ZEP and D1 prevents the reconversion of zeaxanthin during photoinhibition and repair of PSII. This regulation of ZEP activity supports a coordinated degradation of D1 and ZEP during photoinhibition/repair of PSII and an essential photoprotective function of zeaxanthin during the PSII repair cycle.

Differences in isoprenoid-mediated energy dissipation pathways between coastal and interior Douglas-fir seedlings in response to drought

Plants have evolved energy dissipation pathways to reduce photooxidative damage under drought when photosynthesis is hampered. Non-volatile and volatile isoprenoids are involved in non-photochemical quenching of excess light energy and scavenging of reactive oxygen species. A better understanding of trees' ability to cope with and withstand drought stress will contribute to mitigate the negative effects of prolonged drought periods expected under future climate conditions. Therefore we investigated if Douglas-fir (Pseudotsuga menziesii(Mirb.)) provenances from habitats with contrasting water availability reveal intraspecific variation in isoprenoid-mediated energy dissipation pathways. In a controlled drought experiment with 1-year-old seedlings of an interior and a coastal Douglas-fir provenance, we assessed the photosynthetic capacity, pool sizes of non-volatile isoprenoids associated with the photosynthetic apparatus, as well as pool sizes and emission of volatile isoprenoids. We observed variation in the amount and composition of non-volatile and volatile isoprenoids among provenances, which could be linked to variation in photosynthetic capacity under drought. The coastal provenance exhibited an enhanced biosynthesis and emission of volatile isoprenoids, which is likely sustained by generally higher assimilation rates under drought. In contrast, the interior provenance showed an enhanced photoprotection of the photosynthetic apparatus by generally higher amounts of non-volatile isoprenoids and increased amounts of xanthophyll cycle pigments under drought. Our results demonstrate that there is intraspecific variation in isoprenoid-mediated energy dissipation pathways among Douglas-fir provenances, which may be important traits when selecting provenances suitable to grow under future climate conditions.

Nitrogen Supply Affects Photosynthesis and Photoprotective Attributes During Drought-Induced Senescence in Quinoa

Drought during senescence has become more common in Mediterranean climates in recent years. Chenopodium quinoa Willd has been identified as tolerant to poor soil conditions and drought. Previous observations have found that sufficient nitrogen (N) supply mitigates yield losses under terminal drought conditions. However, there is no understanding of the mechanisms behind this effect. We hypothesized that N up-regulates both photosynthetic and photoprotective elements during drought-induced senescence, alleviating the negative impact of drought on yield. The role of N supply and terminal drought on photoprotection was tested using three Chilean quinoa genotypes from different climatic zones: Faro, UdeC9, and BO78. Plants were grown under high nitrogen (HN) or low nitrogen (LN) conditions and subjected to terminal drought at the onset of senescence. Photosynthetic and photochemical and non-photochemical processes were evaluated at both the onset of drought and after 15 days of drought conditions. N supplementation modified most of the physiological parameters related to photochemical dissipation of energy, photosynthesis, and yield in quinoa. In contrast, water restriction did not affect photosynthesis in quinoa, and its effect on yield was dependent on the genotype. A significant interaction N × G was observed in photosynthesis, relative water content, protein content, Fv/Fm, and chlorophylls. In general, Faro was able to maintain higher levels of these attributes under LN conditions than UdeC9 and BO78. In addition, the interacting effects of N × W regulated the level of most pigments in quinoa as well as the photoprotective induction of non-photochemical quenching (NPQ) during senescence. During terminal drought at LN conditions, Faro presented a larger NPQ induction under drought conditions than UdeC9 and BO78, which was supported by a larger zeaxanthin content and de-epoxidation state of the xanthophyll pool. Interestingly, BO78 did not induce NPQ in response to drought-induced senescence but instead enhanced the content of betacyanins. This response needs to be researched in future works. Finally, we observed that LN supply reduced the correlationship between the de-epoxidation state of the xanthophyll cycle and NPQ. This could be an indication that N supply not only compromised the capacity for photosynthetic performance in quinoa plants, but also affected the plasticity of thermal dissipation, restricting further changes during drought-induced senescence.

How do Mediterranean shrub species cope with shade? Ecophysiological response to different light intensities

Under natural conditions, light exposure for Mediterranean shrubs can be highly variable, especially during cloudy days or under a canopy, and can interfere with other environmental factors such as temperature and water availability. With the aim of decoupling the effect of radiation and temperature from water availability, we conducted an experiment where two perennial and three summer semi-deciduous shrub species were subjected to different levels of irradiation. In order to follow plant responses to light exposure, we measured gas exchange, photosystem II photochemical efficiency, photosynthetic pigments and leaf mass area in spring and summer. Results showed that all study species presented a plastic response to different light conditions, and that light-related traits varied in a coordinated manner. Summer semi-deciduous species exhibited a more opportunistic response, with higher photosynthesis rates in full sun, but under shade conditions, the two strategies presented similar assimilation rates. Stomatal conductance did not show such a drastic response as photosynthetsis, being related to changes in WUE. Daily cycles of F_v /F_m revealed a slight photoinhibitory response during summer, mainly in perennial species. In all cases photosynthetic pigments adjusted to the radiation level; leaves had lower chlorophyll content, higher pool of xanthophylls and higher proportion of the de-epoxydaded state of xanthophylls under sun conditions. Lutein content increased in relation to the xanthophyll pool under shade conditions. Our results evidenced that radiation is an important driving factor controlling morphological and physiological status of Mediterranean shrub species, independently of water availability. Summer semi-deciduous species exhibit a set of traits with higher response variability, maximising their photosynthetic assimilation under different sun conditions.

Photoprotective, excited-state quenching mechanisms in diverse photosynthetic organisms

Light-harvesting complexes (LHCs) serve a dual role in photosynthesis, depending on the prevailing light conditions. In low light, they ensure photosynthetic efficiency by maximizing the light absorption cross-section and subsequent energy storage. Under excess light conditions, LHCs perform photoprotective quenching functions to prevent harmful chemical species such as triplet chlorophyll and singlet oxygen from forming and damaging the photosynthetic apparatus. In this Minireview, various photoprotective quenching mechanisms that have been identified in different photosynthetic organisms are surveyed and summarized, and implications for improving photosynthetic productivity are briefly discussed.

Engineering the lutein epoxide cycle into Arabidopsis thaliana

Although sunlight provides the energy necessary for plants to survive and grow, light can also damage reaction centers of photosystem II (PSII) and reduce photochemical efficiency. To prevent damage, plants possess photoprotective mechanisms that dissipate excess excitation. A subset of these mechanisms is collectively referred to as NPQ, or nonphotochemical quenching of chlorophyll a fluorescence. The regulation of NPQ is intrinsically linked to the cycling of xanthophylls that affects the kinetics and extent of the photoprotective response. The violaxanthin cycle (VAZ cycle) and the lutein epoxide cycle (LxL cycle) are two xanthophyll cycles found in vascular plants. The VAZ cycle has been studied extensively, owing in large part to its presence in model plant species where mutants are available to aid in its characterization. In contrast, the LxL cycle is not found in model plants, and its role in photosynthetic processes has been more difficult to define. To address this challenge, we introduced the LxL cycle into Arabidopsis thaliana and functionally isolated it from the VAZ cycle. Using these plant lines, we showed an increase in dark-acclimated PSII efficiency associated with Lx accumulation and demonstrated that violaxanthin deepoxidase is responsible for the light-driven deepoxidation of Lx. Conversion of Lx to L was reversible during periods of low light and occurred considerably faster than rates previously described in nonmodel species. Finally, we present clear evidence of the LxL cycle's role in modulating a rapid component of NPQ that is necessary to prevent photoinhibition in excess light.

Dissecting and modeling zeaxanthin- and lutein-dependent nonphotochemical quenching in Arabidopsis thaliana

Photosynthetic organisms use various photoprotective mechanisms to dissipate excess photoexcitation as heat in a process called nonphotochemical quenching (NPQ). Regulation of NPQ allows for a rapid response to changes in light intensity and in vascular plants, is primarily triggered by a pH gradient across the thylakoid membrane (∆pH). The response is mediated by the PsbS protein and various xanthophylls. Time-correlated single-photon counting (TCSPC) measurements were performed on Arabidopsis thaliana to quantify the dependence of the response of NPQ to changes in light intensity on the presence and accumulation of zeaxanthin and lutein. Measurements were performed on WT and mutant plants deficient in one or both of the xanthophylls as well as a transgenic line that accumulates lutein via an engineered lutein epoxide cycle. Changes in the response of NPQ to light acclimation in WT and mutant plants were observed between two successive light acclimation cycles, suggesting that the character of the rapid and reversible response of NPQ in fully dark-acclimated plants is substantially different from in conditions plants are likely to experience caused by changes in light intensity during daylight. Mathematical models of the response of zeaxanthin- and lutein-dependent reversible NPQ were constructed that accurately describe the observed differences between the light acclimation periods. Finally, the WT response of NPQ was reconstructed from isolated components present in mutant plants with a single common scaling factor, which enabled deconvolution of the relative contributions of zeaxanthin- and lutein-dependent NPQ.

The diatom Phaeodactylum tricornutum adjusts nonphotochemical fluorescence quenching capacity in response to dynamic light via fine-tuned Lhcx and xanthophyll cycle pigment synthesis

Diatoms contain a highly flexible capacity to dissipate excessively absorbed light by nonphotochemical fluorescence quenching (NPQ) based on the light-induced conversion of diadinoxanthin (Dd) into diatoxanthin (Dt) and the presence of Lhcx proteins. Their NPQ fine regulation on the molecular level upon a shift to dynamic light conditions is unknown. We investigated the regulation of Dd + Dt amount, Lhcx gene and protein synthesis and NPQ capacity in the diatom Phaeodactylum tricornutum after a change from continuous low light to 3 d of sine (SL) or fluctuating (FL) light conditions. Four P. tricornutum strains with different NPQ capacities due to different expression of Lhcx1 were included. All strains responded to dynamic light comparably, independently of initial NPQ capacity. During SL, NPQ capacity was strongly enhanced due to a gradual increase of Lhcx2 and Dd + Dt amount. During FL, cells enhanced their NPQ capacity on the first day due to increased Dd + Dt, Lhcx2 and Lhcx3; already by the second day light acclimation was accomplished. While quenching efficiency of Dt was strongly lowered during SL conditions, it remained high throughout the whole FL exposure. Our results highlight a more balanced and cost-effective photoacclimation strategy of P. tricornutum under FL than under SL conditions.

Exogenous Melatonin Mitigates Photoinhibition by Accelerating Non-photochemical Quenching in Tomato Seedlings Exposed to Moderate Light during Chilling

Melatonin plays an important role in tolerance to multiple stresses in plants. Recent studies have shown that melatonin relieves photoinhibition in plants under cold stress; however, the mechanisms are not fully understood. Non-photochemical quenching (NPQ) is a key process thermally dissipating excess light energy that plants employ as a protective mechanism to prevent the over reduction of photosystem II. Here, we report the effects of exogenous melatonin on NPQ and mitigation of photoinhibition in tomato seedlings exposed to moderate light during chilling. In response to moderate light during chilling, the maximum quantum yield (Fv/Fm) and the effective photochemical efficiency (F'v/F'm) of PSII were both substantially reduced, showing severe photoinhibition in tomato seedlings, whereas exogenous application of melatonin effectively alleviated the photoinhibition. Further experiment showed that melatonin accelerated the induction of NPQ in response to moderate light and maintained higher level of NPQ upon longer exposure to light during chilling. Consistent with the increased NPQ was the elevated de-epoxidation state of xanthophyll pigments in melatonin-pretreated seedlings exposed to light during chilling. Enzyme activity assay showed that violaxanthin de-epoxidase (VDE), which catalyzes the de-epoxidation reaction in the xanthophyll cycle, was activated by light and the activity was further enhanced by application of melatonin. Further analysis revealed that melatonin induced the expression of VDE gene in tomato seedlings under moderate light and chilling conditions. Ascorbic acid is an essential cofactor of VDE and the level of it was found to be increased in melatonin-pretreated seedlings. Feeding tomato seedlings with dithiothreitol, an inhibitor of VDE, blocked the effects of melatonin on the de-epoxidation state of xanthophyll pigments and the induction of NPQ. Collectively, these results suggest that exogenous melatonin mitigates photoinhibition by accelerating NPQ through the stimulation of VDE activity and the enhancement of de-epoxidation state of xanthophyll pigments.

The Dynamics of Energy Dissipation and Xanthophyll Conversion in Arabidopsis Indicate an Indirect Photoprotective Role of Zeaxanthin in Slowly Inducible and Relaxing Components of Non-photochemical Quenching of Excitation Energy

The dynamics of non-photochemical quenching (NPQ) of chlorophyll fluorescence and the dynamics of xanthophyll conversion under different actinic light conditions were studied in intact leaves of Arabidopsis thaliana. NPQ induction was investigated during up to 180 min illumination at 450, 900, and 1,800 μmol photons m^-2 s^-1 (μE) and NPQ relaxation after 5, 30, 90, or 180 min of pre-illumination at the same light intensities. The comparison of wild-type plants with mutants affected either in xanthophyll conversion (npq1 and npq2) or PsbS expression (npq4 and L17) or lumen acidification (pgr1) indicated that NPQ states with similar, but not identical characteristics are induced at longer time range (15-60 min) in wild-type and mutant plants. In genotypes with an active xanthophyll conversion, the dynamics of two slowly (10-60 min) inducible and relaxing NPQ components were found to be kinetically correlated with zeaxanthin formation and epoxidation, respectively. However, the extent of NPQ was independent of the amount of zeaxanthin, since higher NPQ values were inducible with increasing actinic light intensities without pronounced changes in the zeaxanthin amount. These data support an indirect role of zeaxanthin in pH-independent NPQ states rather than a specific direct function of zeaxanthin as quencher in long-lasting NPQ processes. Such an indirect function might be related to an allosteric regulation of NPQ processes by zeaxanthin (e.g., through interaction of zeaxanthin at the surface of proteins) or a general photoprotective function of zeaxanthin in the lipid phase of the membrane (e.g., by modulation of the membrane fluidity or by acting as antioxidant). The found concomitant down-regulation of zeaxanthin epoxidation and recovery of photosystem II activity ensures that zeaxanthin is retained in the thylakoid membrane as long as photosystem II activity is inhibited or down-regulated. This regulation supports the view that zeaxanthin can be considered as a kind of light stress memory in chloroplasts, allowing a rapid reactivation of photoprotective NPQ processes in case of recurrent light stress periods.

Transient expression in Nicotiana benthamiana for rapid functional analysis of genes involved in non-photochemical quenching and carotenoid biosynthesis

Plants must switch rapidly between light harvesting and photoprotection in response to environmental fluctuations in light intensity. This switch can lead to losses in absorbed energy usage, as photoprotective energy dissipation mechanisms can take minutes to hours to fully relax. One possible way to improve photosynthesis is to engineer these energy dissipation mechanisms (measured as non-photochemical quenching of chlorophyll a fluorescence, NPQ) to induce and relax more quickly, resulting in smaller losses under dynamic light conditions. Previous studies aimed at understanding the enzymes involved in the regulation of NPQ have relied primarily on labor-intensive and time-consuming generation of stable transgenic lines and mutant populations - approaches limited to organisms amenable to genetic manipulation and mapping. To enable rapid functional testing of NPQ-related genes from diverse organisms, we performed Agrobacterium tumefaciens-mediated transient expression assays in Nicotiana benthamiana to test if NPQ kinetics could be modified in fully expanded leaves. By expressing Arabidopsis thaliana genes known to be involved in NPQ, we confirmed the viability of this method for studying dynamic photosynthetic processes. Subsequently, we used naturally occurring variation in photosystem II subunit S, a modulator of NPQ in plants, to explore how differences in amino acid sequence affect NPQ capacity and kinetics. Finally, we functionally characterized four predicted carotenoid biosynthesis genes from the marine algae Nannochloropsis oceanica and Thalassiosira pseudonana and examined the effect of their expression on NPQ in N. benthamiana. This method offers a powerful alternative to traditional gene characterization methods by providing a fast and easy platform for assessing gene function in planta.

Photoperiod and temperature constraints on the relationship between the photochemical reflectance index and the light use efficiency of photosynthesis in Pinus strobus

The photochemical reflectance index (PRI) is a proxy for the activity of the photoprotective xanthophyll cycle and photosynthetic light use efficiency (LUE) in plants. Evergreen conifers downregulate photosynthesis in autumn in response to low temperature and shorter photoperiod, and the dynamic xanthophyll cycle-mediated non-photochemical quenching (NPQ) is replaced by sustained NPQ. We hypothesized that this shift in xanthophyll cycle-dependent energy partitioning during the autumn is the cause for variations in the PRI-LUE relationship. In order to test our hypothesis, we characterized energy partitioning and pigment composition during a simulated summer-autumn transition in a conifer and assessed the effects of temperature and photoperiod on the PRI-LUE relationship. We measured gas exchange, chlorophyll fluorescence and leaf reflectance during the photosynthetic downregulation in Pinus strobus L. seedlings exposed to low temperature/short photoperiod or elevated temperature/short photoperiod conditions. Shifts in energy partitioning during simulated autumn were observed when the pools of chlorophylls decreased and pools of photoprotective carotenoids increased. On a seasonal timescale, PRI was controlled by carotenoid pool sizes rather than xanthophyll cycle dynamics. Photochemical reflectance index variation under cold autumn conditions mainly reflected long-term pigment pool adjustments associated with sustained NPQ, which impaired the PRI-LUE relationship. Exposure to warm autumn conditions prevented the induction of sustained NPQ but still impaired the PRI-LUE relationship. We therefore conclude that alternative zeaxanthin-independent NPQ mechanisms, which remain undetected by the PRI, are present under both cold and warm autumn conditions, contributing to the discrepancy in the PRI-LUE relationship during autumn.

Diurnal changes in the xanthophyll cycle pigments of freshwater algae correlate with the environmental hydrogen peroxide concentration rather than non-photochemical quenching

BACKGROUND AND AIMS

In photosynthetic organisms exposure to high light induces the production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), which in part is prevented by non-photochemical quenching (NPQ). As one of the most stable and longest-lived ROS, H2O2 is involved in key signalling pathways in development and stress responses, although in excess it can induce damage. A ubiquitous response to high light is the induction of the xanthophyll cycle, but its role in algae is unclear as it is not always associated with NPQ induction. The aim of this study was to reveal how diurnal changes in the level of H2O2 are regulated in a freshwater algal community.

METHODS

A natural freshwater community of algae in a temporary rainwater pool was studied, comprising photosynthetic Euglena species, benthic Navicula diatoms, Chlamydomonas and Chlorella species. Diurnal measurements were made of photosynthetic performance, concentrations of photosynthetic pigments and H2O2. The frequently studied model organisms Chlamydomonas and Chlorella species were isolated to study photosynthesis-related H2O2 responses to high light.

KEY RESULTS

NPQ was shown to prevent H2O2 release in Chlamydomonas and Chlorella species under high light; in addition, dissolved organic carbon excited by UV-B radiation was probably responsible for a part of the H2O2 produced in the water column. Concentrations of H2O2 peaked at 2 µm at midday and algae rapidly scavenged H2O2 rather than releasing it. A vertical H2O2 gradient was observed that was lowest next to diatom-rich benthic algal mats. The diurnal changes in photosynthetic pigments included the violaxanthin and diadinoxanthin cycles; the former was induced prior to the latter, but neither was strictly correlated with NPQ.

CONCLUSIONS

The diurnal cycling of H2O2 was apparently modulated by the organisms in this freshwater algal community. Although the community showed flexibility in its levels of NPQ, the diurnal changes in xanthophylls correlated with H2O2 concentrations. Alternative NPQ mechanisms in algae involving proteins of the light-harvesting complex type and antioxidant protection of the thylakoid membrane by de-epoxidized carotenoids are discussed.

Photochemical and antioxidative responses of the glume and flag leaf to seasonal senescence in wheat

The non-leaf photosynthetic organs have recently attracted much attention for the breeding and screening of varieties of cereal crops to achieve a high grain yield. However, the glume photosynthetic characteristics and responses to high temperature at the late stages of grain filling are not well known in winter wheat (Triticum aestivum L.). In the present study, an experiment was conducted to investigate the anatomy, chloroplast temporal changes, chlorophyll fluorescence, xanthophyll cycle and antioxidative defense system in glumes of field-grown wheat during grain filling compared with flag leaves. Observations using a light microscope revealed that the glumes developed a solid structural base for performing photosynthesis. Compared with the flag leaves, the glumes preserved a more integral ultrastructure, as observed under transmission electron microscopy, and had higher values of Fv/Fm and ΦPSII at the maturity stage. Further analysis of the chlorophyll fluorescence demonstrated that the glumes experienced high non-photochemical quenching (NPQ) at the late stages. Determination of the pool size of the xanthophyll cycle suggested that the (A+Z)/(V+A+Z) ratio was consistently higher in glumes than in flag leaves and that the V+A+Z content was considerably higher in glumes at the maturity stage. In addition, the glumes exhibited a higher antioxidant enzyme activity and a lower accumulation of reactive oxygen species. These results suggest that the glumes are photosynthetically active and senesce later than the flag leaves; the advantages may have been achieved by coordinated contributions of the structural features, higher NPQ levels, greater de-epoxidation of the xanthophyll cycle components and antioxidative defense metabolism.

Climatic origins predict variation in photoprotective leaf pigments in response to drought and low temperatures in live oaks (Quercus series Virentes)

Climate is a major selective force in nature. Exploring patterns of inter- and intraspecific genetic variation in functional traits may explain how species have evolved and may continue evolving under future climate change. Photoprotective pigments play an important role in short-term responses to climate stress in plants but knowledge of their long-term role in adaptive processes is lacking. In this study, our goal was to determine how photoprotective mechanisms, morphological traits and their plasticity have evolved in live oaks (Quercus series Virentes) in response to different climatic conditions. For this purpose, seedlings originating from 11 populations from four live oak species (Quercus virginiana, Q. geminata, Q. fusiformis and Q. oleoides) were grown under contrasting common environmental conditions of temperature (tropical vs temperate) and water availability (droughted vs well-watered). Xanthophyll cycle pigments, anthocyanin accumulation, chlorophyll fluorescence parameters and leaf anatomical traits were measured. Seedlings originating from more mesic source populations of Q. oleoides and Q. fusiformis increased the xanthophyll de-epoxidation state under water-limiting conditions and showed higher phenotypic plasticity for this trait, suggesting adaptation to local climate. Likewise, seedlings originating from warmer climates had higher anthocyanin concentration in leaves under cold winter conditions but not higher de-epoxidation state. Overall, our findings suggest that (i) climate has been a key factor in shaping species and population differences in stress tolerance for live oaks, (ii) anthocyanins are used under cold stress in species with limited freezing tolerance and (iii) xanthophyll cycle pigments are used when photoprotection under drought conditions is needed.

Three causes of variation in the photochemical reflectance index (PRI) in evergreen conifers

The photochemical reflectance index (PRI) reflects diurnal xanthophyll cycle activity and is also influenced by seasonally changing carotenoid : Chl pigment ratios. Both changing pigment pools and xanthophyll cycle activity contribute to photoprotection in evergreen conifers exposed to boreal winters, but they operate over different timescales, and their relative contribution to the PRI signal has often been unclear. To clarify these responses and their contribution to the PRI signal, leaf PRI, pigment composition, temperature and irradiance were monitored over 2 yr for two evergreen conifers (Pinus contorta and Pinus ponderosa) in a boreal climate. PRI was affected by three distinct processes operating over different timescales and exhibiting contrasting spectral responses. Over the 2 yr study period, the greatest change in PRI resulted from seasonally changing carotenoid : Chl pigment ratios, followed by a previously unreported shifting leaf albedo during periods of deep cold. Remarkably, the smallest change was attributable to the xanthophyll cycle. To properly distinguish these three effects, interpretation of PRI must consider temporal context, physiological responses to evolving environmental conditions, and spectral response. Consideration of the separate mechanisms affecting PRI over different timescales could greatly improve efforts to monitor changing photosynthetic activity using optical remote sensing.

Application of heat stress in situ demonstrates a protective role of irradiation on photosynthetic performance in alpine plants

The impact of sublethal heat on photosynthetic performance, photosynthetic pigments and free radical scavenging activity was examined in three high mountain species, Rhododendron ferrugineum, Senecio incanus and Ranunculus glacialis using controlled in situ applications of heat stress, both in darkness and under natural solar irradiation. Heat treatments applied in the dark reversibly reduced photosynthetic performance and the maximum quantum efficiency of photosystem II (Fv /Fm), which remained impeded for several days when plants were exposed to natural light conditions subsequently to the heat treatment. In contrast, plants exposed to heat stress under natural irradiation were able to tolerate and recover from heat stress more readily. The critical temperature threshold for chlorophyll fluorescence was higher under illumination (Tc (')) than in the dark (Tc). Heat stress caused a significant de-epoxidation of the xanthophyll cycle pigments both in the light and in the dark conditions. Total free radical scavenging activity was highest when heat stress was applied in the dark. This study demonstrates that, in the European Alps, heat waves can temporarily have a negative impact on photosynthesis and, importantly, that results obtained from experiments performed in darkness and/or on detached plant material may not reliably predict the impact of heat stress under field conditions.

Relaxation of the non-photochemical chlorophyll fluorescence quenching in diatoms: kinetics, components and mechanisms

Diatoms are especially important microorganisms because they constitute the larger group of microalgae. To survive the constant variations of the light environment, diatoms have developed mechanisms aiming at the dissipation of excess energy, such as the xanthophyll cycle and the non-photochemical chlorophyll (Chl) fluorescence quenching. This contribution is dedicated to the relaxation of the latter process when the adverse conditions cease. An original nonlinear regression analysis of the relaxation of non-photochemical Chl fluorescence quenching, qN, in diatoms is presented. It was used to obtain experimental evidence for the existence of three time-resolved components in the diatom Phaeodactylum tricornutum: qNf, qNi and qNs. qNf (s time-scale) and qNs (h time-scale) are exponential in shape. By contrast, qNi (min time-scale) is of sigmoidal nature and is dominant among the three components. The application of metabolic inhibitors (dithiothreitol, ammonium chloride, cadmium and diphenyleneiodonium chloride) allowed the identification of the mechanisms on which each component mostly relies. qNi is linked to the relaxation of the ΔpH gradient and the reversal of the xanthophyll cycle. qNs quantifies the stage of photoinhibition caused by the high light exposure, qNf seems to reflect fast conformational changes within thylakoid membranes in the vicinity of the photosystem II complexes.

Does plant colour matter? Wax accumulation as an indicator of decline in Juniperus thurifera

The photosynthesis in evergreen trees living in Mediterranean ecosystems is subjected to multiple climatic stresses due to water shortage and high temperatures during the summer and to low temperatures during the winter. Mediterranean perennials deploy different photoprotective mechanisms to prevent damage to the photosynthetic system. Wax accumulation in leaves is a primary response which by enhancing light scattering in the leaf surface reduces incident radiation in the mesophyll. The existence of high variability in wax accumulation levels between coexisting individuals of a species has a visual effect on colour that provides distinguishable green and glaucous phenotypes. We explored this variability in a Mediterranean evergreen tree Juniperus thurifera (L.) to evaluate the impact of epicuticular wax on optical and ecophysiological properties and on the abundance of photoprotective pigments throughout an annual cycle. Because of light attenuation by waxes, we expected that glaucous phenotypes would lower the need for photoprotective pigments. We evaluated the effect of phenotype and season on reflectance, defoliation levels, photochemical efficiency and photoprotective pigment contents in 20 green and 20 glaucous junipers. Contrary to our expectations, the results showed that glaucous trees suffered from a diminution in photochemical efficiency, but there was no reduction in photoprotective pigments. Differences between glaucous and green phenotypes were greater in winter, which is the most stressful season for this species. Glaucous individuals also showed the highest levels of leaf defoliation. The lower photochemical efficiency of glaucous trees, together with higher defoliation rates and equal or greater number of physiological photoprotective mechanisms, suggests that in spite of wax accumulation, glaucous trees suffer from more severe stress than green ones. This result suggests that changes in colouration in Mediterranean evergreens may be a decline indicator.

Assessing leaf photoprotective mechanisms using terrestrial LiDAR: towards mapping canopy photosynthetic performance in three dimensions

Terrestrial laser scanning (TLS) data allow spatially explicit (x, y, z) laser return intensities to be recorded throughout a plant canopy, which could considerably improve our understanding of how physiological processes vary in three-dimensional space. However, the utility of TLS data for the quantification of plant physiological properties remains largely unexplored. Here, we test whether the laser return intensity of green (532-nm) TLS correlates with changes in the de-epoxidation state of the xanthophyll cycle and photoprotective non-photochemical quenching (NPQ), and compare the ability of TLS to quantify these parameters with the passively measured photochemical reflectance index (PRI). We exposed leaves from five plant species to increasing light intensities to induce NPQ and de-epoxidation of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z). At each light intensity, the green laser return intensity (GLRI), narrowband spectral reflectance, chlorophyll fluorescence emission and xanthophyll cycle pigment composition were recorded. Strong relationships between both predictor variables (GLRI, PRI) and both explanatory variables (NPQ, xanthophyll cycle de-epoxidation) were observed. GLRI holds promise to provide detailed (mm) information about plant physiological status to improve our understanding of the patterns and mechanisms driving foliar photoprotection. We discuss the potential for scaling these laboratory data to three-dimensional canopy space.

Seedling ontogeny and environmental plasticity in two co-occurring shade-tolerant conifers and implications for environment-population interactions

PREMISE OF THE STUDY

Seedling success is determined by evolved strategies of intrinsic genetic programming and plasticity that are regulated by extrinsic pathways. We tested the relative importance of these mechanisms in red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea Lin.), which share understory regeneration niches in northeastern North America. Although its reproductive effort is adequate, spruce has decreased in abundance, in relation to fir, in seedling and sapling populations, even in forests that have a predominance of spruce in the overstory.

METHODS

To understand the factors that regulate this phenomenon and their implications for tree populations, we compared intrinsic and plastic regulation of first- and second-year seedlings under steady understory irradiance levels and in response to increases in light environment.

KEY RESULTS

Both species exhibited interactions of ontogenetic patterns and plasticity in first- and second-year seedlings. Physiologically, spruce had higher photosynthetic capacity, allocation to photoprotective xanthophylls, and greater plasticity in response to light treatments. Although both species demonstrated an inability to plastically increase photosynthetic capacity in the short term, spruce benefited from greater allocation to foliage under increased irradiance. Fir showed a conservative strategy in root-shoot allocation that may better equip seedlings to withstand drought adaptations and attributes associated with greater shade tolerance.

CONCLUSIONS

These attributes likely contribute to the relative success of fir seedlings in the current climate. By contrast, they indicate that spruce would be a superior competitor in cooler, moister climates, which suggests that future forest composition will be largely determined by an interaction of disturbance and moisture regimes.

Responses of foliar antioxidative and photoprotective defence systems of trees to drought: a meta-analysis

Current climate change predictions hint to more frequent extreme weather events, including extended droughts, making better understanding of the impacts of water stress on trees even more important. At the individual plant level, stomatal closure as a result of water deficit leads to reduced CO2 availability in the leaf, which can lead to photo-oxidative stress. Photorespiration and the Mehler reaction can maintain electron transport rates under low internal CO2, but result in production of reactive oxygen species (ROS). If electron consumption is decreased, upstream photochemical processes can be affected and light energy is absorbed in excess of photochemical requirements. Trees evolved to cope with excess energy and elevated concentration of ROS by activating photoprotective and antioxidative defence systems. The meta-analysis we present here assessed responses of these defence systems reported in 50 studies. We found responses to vary depending on stress intensity, foliage type and habitat, and on whether experiments were done in the field or in controlled environments. In general, drought increased concentrations of antioxidants and photoprotective pigments. However, severe stress caused degradation of antioxidant concentrations and oxidation of antioxidant pools. Evergreen trees seemed to preferentially reinforce membrane-bound protection systems zeaxanthin and tocopherol, whereas deciduous species showed greater responses in water-soluble antioxidants ascorbic acid and glutathione. Trees and shrubs from arid versus humid habitats vary in their antioxidative and photoprotective defence responses. In field experiments, drought had greater effects on some defence compounds than under controlled conditions.

Deficiency of phytochrome B alleviates chilling-induced photoinhibition in rice

PREMISE OF THE STUDY

Food crops of tropical origins, such as rice, are often cultivated in areas with suboptimal temperature regimes. The rice phytochrome B-deficient mutant (phyB) is tolerant of chilling temperatures compared with the wild type (WT) under low irradiance, and unsaturated fatty acids (USFAs) of membrane lipids have been shown to play an important role in chilling resistance. However, the relationship between phytochrome B and membrane lipids has not been empirically investigated. •

METHODS

We assessed various photosynthesis indexes in phyB and WT rice: chlorophyll content, maximal photochemical efficiency (Fv/Fm) of photosystem II (PSII), the quantum yield of PSII electron transport (ΦPSII), the percentage of oxidizable P700 (P700), nonphotochemical quenching (NPQ), and the de-epoxidized ratio of xanthophyll cycle (A+Z)/(V+A+Z). We also analyzed the ultrastructure and fatty acid desaturases (FADs) and glycerol-3-phosphate acyltransferase (GPAT) genes of the chloroplasts using transmission electron microscopy and real-time PCR. •

RESULTS

After a chilling treatment of 24 h, chloroplast damage and USFA content reduction were more severe in the WT than in the phyB mutant. Genes involved in the synthesis of USFAs in membranes such as FADs and GPAT were more stable in phyB than in WT. Chlorophyll content, Fv/Fm, ΦPSII, and P700 decreased more slowly under chilling stress and recovered more rapidly under optimal conditions in phyB than in WT. The (A+Z)/(V+A+Z) and NPQ increased more rapidly in phyB than in the WT after 24 h of chilling treatment. •

CONCLUSIONS

Phytochrome B deficiency in rice with more stabilized chloroplast structure and higher USFA content in membrane lipids could alleviate chilling-induced photoinhibition.

Occurrence of the PsbS and LhcSR products in the green alga Ulva linza and their correlation with excitation pressure

To avoid photoinhibition, plants have developed diverse photoprotection mechanisms. One of the short-term high light protection mechanisms in plants is non-photochemical quenching (NPQ), which dissipates the absorbed light energy as thermal energy. In the green alga, Ulva linza, the kinetics of NPQ starts with an initial, quick rise followed by a decline, and then a second and higher rise at longer time periods. During the whole phase, NPQ is triggered and controlled by ΔpH, then strengthened and modulated by zeaxanthin. Light-harvesting complex (LHC) family members are known to play crucial roles in this mechanism. The PSBS protein, a member of the LHC family that was thought to be present exclusively in higher plants, has been identified for the first time in U. linza. The expression of both PSBS and LHCSR was up-regulated during high light conditions, and LHCSR increased more than PSBS. Both LHCSR and PSBS-dependent NPQ may be important strategies for adapting to the environment, and they have undoubtedly played a role in their evolution.